If you're among friends, then any plural that is understood is okay (e.g. Rallys, RTA's, &etc). In a slightly more formal case, I would recommend: "Rally the Ancestors cards"or "Rally cards"
Edit/expansion:
In general, the rule for pluralizing difficult nouns is to use the plural form of the base noun. As an example, many people incorrectly pluralize "brother in-law" as "brother in-laws" - i.e. they simply stick an 's' on the end. But the "in-law" portion is simply a modification of the base noun, brother. So the correct plural is "brothers in-law".
Similarly, in your case, the base noun is "card" and all the rest just describes what kind of card you're talking about. Which is why the proper plural would be "Rally the Ancestors cards".